The assignee of the present invention manufactures and deploys spacecraft for, commercial, defense and scientific missions. On board propulsion systems of such spacecraft are frequently required to perform orbit raising (or transfer). For example, there is frequently a requirement for commercial spacecraft to perform orbit raising from a launch vehicle transfer orbit to a geosynchronous orbit. As a further example, certain missions may require transfers between orbits. Such maneuvers may be performed with chemical thrusters, or with one or more with low thrust electric thrusters, as described by Oh, U.S. Pat. No. 6,543,723 (hereinafter “Oh”), assigned to the assignee of the present invention, and Gelon, et al., U.S. Pat. No. 7,113,851, (hereinafter “Gelon”) entitled “Practical Orbit Raising System and Method for Geosynchronous Satellites” assigned to the assignee of the present invention, and Gelon.
Known orbit raising techniques are also described in U.S. Pat. No. 5,595,360 issued to Spitzer, entitled “Optimal Transfer Orbit Trajectory Using Electric Propulsion,” U.S. Pat. No. 6,116,543, issued to Koppel, entitled “Method and a System for Putting a Space Vehicle into Orbit, Using Thrusters of High Specific Impulse.”
Characteristically, during such transfers, spacecraft momentum has to be managed so as to provide three axis attitude control. Momentum storage systems are employed to store accumulated momentum resulting from a disturbance torque environment, and thereby reduce the pointing disturbance and propellant usage associated with a thruster actuation. These systems, consisting of reaction wheels, have a storage capacity that may be described in terms of a permissible range of wheel speeds. As a result, a momentum management strategy must use thrusters or other actuators such as magnetic torquers or solar sailing techniques to unload momentum in order to prevent wheel speeds from going outside the permissible range.
Known orbit raising techniques provide momentum management during long duration operation of electric propulsion thrusters by gimbaling and/or throttling the thruster(s) providing the orbit raising velocity change. Where, as is desirable for reliability and cost reasons, orbit raising is to be performed with thruster(s) mounted on a single positioning mechanism, a problem arises that such a single gimbaled thruster can only provide torque about the two axes orthogonal to its thrust axis. Thus, it is not possible to generate torque parallel to the thrust vector. Conventionally, this problem is solved by providing at least one additional actuator to provide yaw authority.
As a result, system performance is penalized by the additional hardware cost, mass, and complexity.